Dca7 (S. pombe, O74763) — Core-Function Hypothesis Review

Hypothesis: Schizosaccharomyces pombe Dca7 forms an evolutionarily conserved scaffold complex with a DYRK/Yak-family kinase.
Focus type: core_function • Gene: dca7 / SPBC17D11.08 / UniProt O74763 (YBE8_SCHPO) • Iteration: 1 of 3

Executive Judgment

Verdict: Supported (strong for orthology/conservation; moderate for direct in‑organism demonstration).

The hypothesis is well supported by two independent lines of evidence that converge:

1. Orthology/evolutionary evidence (strong). Dca7 is the fission‑yeast ortholog of human DCAF7/WDR68 (InterPro IPR045159 "DCAF7‑like"; PomBase 1:1 orthologs = human DCAF7 HGNC:30915 and S. cerevisiae YPL247C). WDR68/DCAF7 is a textbook, deeply conserved seven‑bladed WD40 β‑propeller scaffold for DYRK‑family kinases (DYRK1A/DYRK1B), documented in humans, zebrafish, insects and plants (PMID 21777625, 23349862, 25342745).
2. Direct in‑organism interaction (moderate). In S. pombe, Dca7 physically binds ppk15 (SPAC823.03), whose catalytic domain is specifically the Yak1‑type kinase domain (CDD cd14212 PKc_YAK1) and whose orthologs are human DYRK1A/DYRK1B and budding‑yeast Yak1 (YJL141C). The Dca7–ppk15 interaction is a verified binary yeast‑two‑hybrid hit from the fission‑yeast interactome (Vo et al., Cell 2016; PMID 26771498).

Thus the two fission‑yeast orthologs of the conserved human DCAF7–DYRK1A/1B pair physically associate in S. pombe — precisely the relationship the seed hypothesis predicts.

Three‑species concordance (added Iteration 2). STRING confirms the DCAF7–Yak/DYRK partnership independently in two yeasts: S. cerevisiae Yak1–YPL247C combined score 0.996 (experimental subscore 0.979) and S. pombe dca7–ppk15 combined 0.933 (experimental subscore 0.873). Together with the direct mammalian DCAF7/WDR68–DYRK1A/1B binding (PMID 21777625), the kinase–scaffold pairing is experimentally supported across human, budding‑yeast and fission‑yeast lineages.

Sequence-conservation provenance (added Iteration 3). Global Needleman–Wunsch/BLOSUM62 alignments computed this run: Dca7 vs human DCAF7 = 44.2% identity (328 aligned cols), Dca7 vs Sc YPL247C = 37.5%, and human DCAF7 vs YPL247C = 42.7%. This ~38–44% identity across the shared ~330‑residue WD40 β‑propeller core, preserved over ~1 billion years, confirms Dca7 is a bona‑fide DCAF7/WDR68 ortholog (not a name‑only match). See artifacts/conservation_provenance.md, artifacts/evidence_matrix.csv, artifacts/go_decision_table.csv.

GO annotation state (added Iteration 2). QuickGO for O74763 shows the molecular‑function and biological‑process roots are annotated ND ("no data," GO_REF:0000015) — Dca7 has no experimental MF or BP annotation at all. The only complex CC term, GO:0080008 "Cul4‑RING E3 ubiquitin ligase complex," is evidence code ISS, projected by similarity from human DCAF7 (UniProtKB:P61962). Nucleus (GO:0005634) is IBA (phylogenetic); cytoplasm/Golgi are IEA (UniProt‑SubCell). So the DYRK/Yak‑scaffold role is a genuine, unfilled annotation gap.

Most important caveats. (i) The pombe interaction rests on a single high‑throughput Y2H method, without in‑organism co‑IP or a demonstrated scaffolding function (PomBase characterisation status = "conserved unknown"). (ii) "Scaffold" is an inference from orthology, not a measured pombe activity. (iii) The Cul4‑RING E3 complex CC annotation is an ISS carry‑over from human DCAF7, not pombe evidence, and is a plausible over‑annotation relative to the better‑supported DYRK‑scaffold role.

Evidence Matrix

GO Curation Implications (leads — require curator verification)

• Molecular Function (add — currently ND). dca7's MF root is annotated ND ("no data"), so there is no existing MF to retain or conflict with. Evidence supports adding a kinase‑binding MF. Recommended lead: GO:0019901 "protein kinase binding" (or the more specific protein serine/threonine kinase binding), evidence IPI from PMID 26771498 (dca7↔ppk15) supported by orthology (ISO to DCAF7–DYRK1A/1B). More informative than "protein binding" and should be preferred. No "DYRK binding" child term exists; "protein kinase binding" is the best‑supported informative term.
• Cellular Component (review; over‑annotation confirmed as ISS carry‑over). QuickGO shows GO:0080008 "Cul4‑RING E3 ubiquitin ligase complex" is evidence code ISS, projected by similarity from human DCAF7 (UniProtKB:P61962) — not pombe experimental data. In metazoa WDR68 does associate with CUL4‑DDB1 (PMID 25342745), so it is not wrong in principle, but for S. pombe it should be treated as not experimentally established / non‑core and flagged for review. The nucleus (GO:0005634) term is IBA (phylogenetic), and UniProt independently annotates cytoplasm + Golgi (IEA), so the subcellular picture is inference‑only and partner‑dependent; reconcile rather than assert nuclear localization. A Dca7–ppk15 complex CC could be a future lead but currently lacks a defined complex term.
• Biological Process (candidate). Consider a "regulation of DYRK/Yak kinase signaling"‑type BP by orthology, but keep as non‑core/tentative until pombe function is shown. Δdca7 phenotypes (G0/stationary‑phase viability, glucose‑starvation growth) are downstream/pleiotropic and should not be used to assign a direct MF/BP.
• Net recommendation: The seed hypothesis should inform the review by adding a kinase‑binding MF lead and by scrutinizing the CUL4 CC annotation. Do not upgrade "scaffold" to an asserted pombe function without functional data.

Mechanistic Scope

• Immediate molecular function being tested: direct physical binding of the Dca7 WD40 β‑propeller to a DYRK/Yak‑family kinase (ppk15), i.e., a kinase‑scaffold/adaptor activity.
• Direct (supported): Dca7 = DCAF7/WDR68 ortholog; Dca7 binds ppk15 (Yak/DYRK ortholog) in pombe; the human orthologous pair binds directly and the interaction is conserved.
• Downstream / not the tested activity: Δdca7 growth and stress phenotypes; "implicated in gene expression"; anthocyanin/craniofacial/developmental outcomes of orthologs; possible CUL4‑DDB1 ubiquitin‑ligase adaptor role. These are pathway consequences or context‑specific roles, not the core scaffold activity.

Conflicts and Alternatives

• Competing molecular role (CUL4 adaptor). The "DCAF7 = DDB1‑CUL4 associated factor" identity and the pombe CC annotation to a Cul4‑RING E3 complex offer an alternative/additional function. Literature (PMID 25342745) confirms WDR68–CUL4‑DDB1 association in metazoa, so this is a genuine dual role, not simply an error — but which role is "core" in pombe is unresolved. The DYRK‑scaffold role is the more consistently documented, defining function across the family.
• Paralog/kinase‑identity nuance. S. pombe has more than one CMGC/DYRK‑type kinase (e.g., pom1, MNB/DYRK subfamily; lkh1, LAMMER; prp4). The kinase captured interacting with Dca7 is specifically ppk15 (Yak1/DYRK1A/1B ortholog), the expected DCAF7 partner — this argues against paralog confusion, but a curator should confirm ppk15 (not pom1) is the relevant partner.
• Localization conflict. UniProt (cytoplasm + Golgi) vs PomBase GO CC (nucleus) — WDR68 orthologs are pan‑cellular and shuttle with their partners (PMID 21777625, 23349862), so both may be partially correct in a partner‑dependent manner.
• Method limitation. A single Y2H hit could in principle be a false positive; however, dca7 has only two recorded physical partners (not a sticky hub), which raises confidence.

Knowledge Gaps

1. In‑organism validation of Dca7–ppk15. Checked: only one HT Y2H (PMID 26771498). Matters because "scaffold" claims need biochemical confirmation. Resolve with co‑IP / affinity‑MS of tagged Dca7 in pombe and reciprocal ppk15 pulldown.
2. Functional scaffolding readout. Checked: PomBase status "conserved unknown"; no pombe assay. Matters for MF/BP assignment. Resolve by testing whether Δdca7 alters ppk15 localization, stability, or substrate phosphorylation (analogous to WDR68 controlling DYRK1A nuclear accumulation).
3. Budding‑yeast anchor. Largely resolved (Iteration 2): STRING shows Yak1–YPL247C with combined 0.996 / experimental 0.979, strongly supporting the conserved pairing in S. cerevisiae. Remaining gap: the specific primary reference(s) behind that experimental subscore were not individually confirmed (likely large AP‑MS/PCA datasets). Resolve via BioGRID/SGD YPL247C interaction records.
4. CUL4 CC evidence code. Resolved (Iteration 2): GO:0080008 is ISS from human DCAF7 (P61962) — a similarity carry‑over, confirming the over‑annotation concern. Curator action: decide whether to retain as non‑core ISS or remove pending pombe evidence.

Discriminating Tests

• Co‑IP / proximity‑labeling (BioID/TurboID) of Dca7 in S. pombe to confirm ppk15 (and to distinguish ppk15 vs pom1 as the partner).
• AlphaFold‑Multimer / AF3 modeling of Dca7–ppk15 vs Dca7–DDB1 to compare interface plausibility (a fast in‑silico discriminator between the DYRK‑scaffold and CUL4‑adaptor hypotheses).
• Structure‑guided mutagenesis of the Dca7 β‑propeller "top face" (the DYRK‑binding surface in DCAF7) to test whether it is required for ppk15 binding and for Δdca7 phenotypes.
• Phospho‑proteomics of Δdca7 vs WT to test whether Dca7 modulates ppk15/Yak‑dependent phosphorylation (functional scaffolding).
• Cross‑complementation: can human DCAF7 or the Dca7–ppk15 pair reconstitute known DYRK‑scaffold outputs?

Curation Leads (require curator verification)

• Candidate references to verify:
• PMID 26771498 (Vo et al., Cell 2016) — verify the exact Dca7↔ppk15 binary Y2H record in the supplementary interactome / BioGRID.
• PMID 21777625 (Miyata & Nishida 2011) — snippet: "DYRK1A and DYRK1B, but not DYRK2, DYRK3, or DYRK4, bound to endogenous and expressed WDR68."
• PMID 23349862 (Wang et al. 2013) — snippet: "Wdr68 is a highly conserved scaffolding protein… A Ras‑Map3k‑Wdr68‑Dyrk1 signaling relay…"
• PMID 25342745 (Miyata et al. 2014) — snippet: "WDR68 is an evolutionarily conserved WD40 repeat protein that binds to several proteins, including … (DYRK1A), … (MEKK1), and Cullin4‑damage‑specific DNA‑binding protein 1 (CUL4‑DDB1)."
• Candidate GO term changes:
• Add MF: GO:0019901 protein kinase binding (or protein serine/threonine kinase binding) — evidence IPI (PMID 26771498) + ISO (DCAF7). Preferred over "protein binding."
• Review CC: GO:0080008 Cul4‑RING E3 ubiquitin ligase complex — flag as name/orthology‑based; confirm evidence code; consider demoting to non‑core for pombe. Reconcile nucleus (GO) vs cytoplasm/Golgi (UniProt).
• Optional BP (tentative, non‑core): regulation of DYRK/Yak kinase signaling — by orthology only.
• Suggested questions for curator: Is ppk15 (not pom1) the intended DYRK/Yak partner? Is the Cul4 complex annotation experimentally grounded in pombe or an IEA carry‑over? Should Dca7's core function be recorded as "DYRK‑kinase scaffold/adaptor" pending functional data?
• Suggested experiments: pombe co‑IP/BioID of Dca7; AF‑Multimer Dca7–ppk15; Δdca7 phospho‑proteomics; propeller‑surface mutants.

Provenance

All database results were retrieved programmatically this run: UniProt REST (O74763), PomBase API (SPBC17D11.08, SPAC823.03, SPAC25A8.01c), InterPro/CDD annotations, and NCBI eSummary for PMIDs. Interaction counts and orthology mappings are quoted directly from those queries. Human functional claims are from the cited primary papers (PMIDs above). No results were fabricated; where a resource was not queried (e.g., SGD YPL247C–Yak1), it is listed as a knowledge gap.

Artifacts

• OpenScientist conservation provenance
• OpenScientist evidence matrix
• OpenScientist go decision table
• OpenScientist final report
• OpenScientist final report